Methanol has the potential to become an important energy vector. The so-called "methanol economy" is essentially a carbon-neutral cycle consisting of two groups of reactions: Methanol synthesis from a mixture of CO, CO2, and H2, and methanol-to-hydrogen conversion reaction. Latter includes methanol decomposition (or dry dehydrogenation), partial oxidation, steam reforming, and autothermal reforming. Cu-based materials are currently our best option as catalysts, and they are already used industrially in some of the above-mentioned reactions. This makes the interface between Cu and methanol vapor exceedingly important. Yet, only a handful of experimental mechanistic studies (at the molecular level), under realistic reaction conditions, are available in the literature. Herein, the interaction and decomposition of methanol on different copper surface orientations, Cu(111), Cu(100) and Cu(110), have been studied by means of PM-IRRAS and AP-XPS under 1 mbar methanol pressure in the temperature range of 25-100 °C. Our results reveal that methanol is dissociatively adsorbed on the clean Cu surfaces to form methoxy (CH3O*) and hydrogen at ambient conditions. The temporal evolution of infrared spectra indicates that a transient state of a high-coverage methoxy layer forms immediately after methanol exposure. For achieving an equilibrium coverage, the methoxy excess is eliminated via a further dehydrogenation to CO and its desorption to the gas phase. The kinetics of this process, which involves the activation of C-H bonds, displays a significant structure sensitivity with a much faster kinetics on the corrugated Cu(110) compared to the close-packed surfaces of Cu(111) and Cu(100). We also propose a model that explains the origin of the initial metastable methoxy coverage by considering the previous step of molecular adsorption in the form of H-bonded assemblies.

Methanol decomposition on copper surfaces under ambient conditions: Mechanism, kinetics and structure sensitivity

ACS Fall 2021

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